1. Field of the Invention
The present invention is related an analog-digital converter and an analog-digital converting method for outputting the analog-digital conversion codes without undesirable enhanced effect of noise mixed to the input analog signal, and also related to a volume controller system designed with such an analog-digital converter.
2. Prior Art
FIGS. 1(a) and 1(b) are schematic diagrams showing a prior art analog-digital converter with the resolution of 3 bits in which FIG. 1(a) is a view showing the overall configuration thereof while FIG. 1(b) is a view showing the circuit diagram of the analog switch.
The analog-digital converter is composed of a conversion level output circuit 101, a decoder (3 to 8) 102, the comparator 103, a resettable D-type flip-flop 104, the 3-bit serial-in-parallel-out (SIPO) register 105, the 3-bit parallel-in-parallel-out (PIPO) register 106 and the clock signal generator 107, as illustrated in FIG. 1(a).
The conversion level output circuit 101 is composed of a plurality of resistors connected in serial between a power source line VDD and the ground GND for generating the conversion potential levels respectively corresponding to the analog-digital conversion codes of [111] to [000], and a series of analog switches SW111 to SW001 connected to the connection points of the series of resistors.
For example, the analog switches SW001.about.SW111 are composed of a Pch-MOS transistor 111, an Nch-MOS transistor 112 and an inverter 113 as illustrated in FIG. 1(b), and turned on/off in accordance with the potential levels of the clock signal CK1 to CK3, as illustrated in FIG. 2, under the control of the decoder 102.
FIG. 3 is a timing chart showing the operation of the analog-digital converter.
In accordance with the operation of the analog-digital converter, one of the analog switches SW111 to SW001 is turned on in sequence, the output voltage thereof and the signal level INPUT of the analog signal as inputted are compared with each other by means of the comparator 103.
When the conversion potential level as outputted from the conversion level output circuit 101 becomes lower than the signal level INPUT of the analog signal as inputted, the comparison output level CO of the comparator 103 rises to the "H" level.
The output signal COD of the D-type flip-flops 104 rises then to the "H" level, followed by the three-bit output signal of the register 105 being [010]. The above data [010] are transferred to the register 106 in a "L" to "H" timing of the LD signal, and the analog-digital conversion code [010] is outputted as the output signal OUTPUT of the analog-digital converter.
The analog-digital converting operation is repeated followed by outputting the analog-digital conversion code [001] as the output signal OUTPUT of the analog-digital converter.
Next, a volume controller system designed with the analog-digital converter as described above will be explained in the followings.
FIG. 4 is a schematic diagram showing such a system.
The volume controller system is provided with the analog-digital converter 120 as illustrated in FIG. 1 for the purpose of controlling a digital volume controller 122 by the digitized signal converted from the DC potential outputted by the external volume controller 121. The conversion is performed by the analog-digital converter 120.
For example, the analog-digital converter 120 and the digital volume controller 122 are integrated in an audio controlling IC chip while the volume controller 121 is provided external to the IC chip.
FIG. 5 is an exemplary circuit diagram showing the digital volume controller 122. In this case, the digital volume controller 122 is an 8-level volume controller composed of the decoder (3 bits to 8 levels) and an output voltage generation circuit 122b. The relationship between the three-bit input data INPUT (A,B,C) and the three-bit output data OUTPUT is illustrated in FIG. 6. On the other hand, the output voltage generation circuit 122b is composed of a plurality of resistors "r" and a plurality of switches SW0 to SW7.
The 8-level volume controller 122 can be controlled by three-bit data, for which the analog-digital converter as illustrated in FIG. 1 can be used for implementing the volume controller system as illustrated in FIG. 4.
In the case of the volume controller system, when the input level (4V) as illustrated in FIG. 7, the analog-digital conversion code [110] is outputted from the analog-digital converter 120. The output signal S3 of the decoder 122 as illustrated in FIG. 5 becomes "H" with the switch SW3 being turned on. As a result, the signal inputted to the IN terminal is attenuated to 4/7 and outputted to the OUT terminal.
However, there are following shortcomings in the prior art analog-digital converter.
As illustrated in FIG. 7, in the case that the input level B (2.5V) is inputted as the DC potential, the analog-digital conversion code may often fluctuate between the analog-digital conversion codes [011] and [100], because the input level is between the adjacent conversion regions.
More specifically explained, as illustrated in FIG. 8(a), the signal input to the analog-digital converter 120 is necessarily mixed with noise so that, when a signal is input in the vicinity of the boundary, the analog-digital conversion code as output necessarily fluctuates in accordance with the fluctuation of the noise.
For example, when noise as mixed has a triangle wave form P11 as illustrated in FIG. 8(b), the output signal CSD(OUTPUT) of the analog-digital converter 120 periodically fluctuates between the output levels A and B ([011],[100]). As a result, when a continuous audio signal is input to the IN terminal of the digital volume controller 122, the output signal becomes modulated as treated by amplitude modulation. For this reason, the output sound sometimes becomes deteriorated and discomfortable. While the DC control signal level (the signal level of the analog signal as inputted) not frequently occurs at some of the boundaries, the greater the noise level the greater the probability of occurrence of the shortcomings because the fluctuation range is expanded with greater noise.
Accordingly, in the case of the prior art analog-digital converter, when a signal is input in the vicinity of the boundary, the analog-digital conversion code as output fluctuates due to the noise mixed to the input analog signal.